Heat-recoverable or heat reactive articles are used in a variety of commercial areas, and are typically based on the concept that the article, upon the application of heat thereto, will shrink to a predetermined position One area where such articles have exemplary utility is in the repair of telecommunication cables. The heat-recoverable article can be applied to the proper cable location, followed by heating to cause shrinkage thereof around the cable to insure a tight fit without the availability of ingress of moisture. Additionally, there is typically present on the interior surface of such articles a meltable sealant or hot melt adhesive which melts and flows during the shrinking process to aid in the sealing of the article and the cable to degrative environments. Obviously, it is incumbent that the heat-recoverable article be heated sufficiently to cause proper shrinkage and sealant flow, yet not excessively heated so as to incur degradation of the typically polymeric recoverable material.
Accordingly, it is desirable to have included with the article a thermochromic material which will undergo a color change upon achievement of a predetermined temperature.
Thermochromic pigments based on inorganic compounds, typically transition metal compounds, were initially commercially available on some heat reactive articles in the trade. In U.S. Pat. Nos. 4,105,583 and 4,121,011, for example, there are disclosed improvements designed to slow the degradation of heat reactive substrates. However, such materials based on transition metal compounds have resulted in problems, such as degradation of the heat reactive plastic substrates upon thermal conversion thereof, possibly due to flame propagation; minimal visibility of the thermal conversion; moisture sensitivity of the compositions; a reversibility tendency, i.e., a tendency to revert back to the original form subsequent to thermal conversion; and poor pigment dispersibility in the paint composition.
Organic compounds which are taught to melt and decompose are disclosed as thermochromics for use with heat reactive articles, typically with binders which prevent the leaching out thereof, in U.S. Pat. No. 4,344,909; U.K. Patent Application No. 2,038,478A; and U.K. Patent Application No. 2,077,919A. Further, EPO Patent No. WO84/00608 discloses a two component organic system wherein a colorant and an activator are used in an ultraviolet cured binder system on heat reactive articles. By varying the amount of the activator, it is taught that the temperature of conversion can similarly be varied.
Thermochromics based on such organic pigments suffer many of the problems noted above, i.e., moisture sensitivity/water leachability of pigments and/or degradation products; difficult dispersibility of pigments; poor hiding power, because of the relatively low refractive index of such pigments. Furthermore, a true white-to-black conversion may not be attainable because the initial color of typical pigments is an off-white masked by a green or blue dye. In some cases the final converted color may be brown instead of black, thus not providing optimum visual indication of conversion.
It has been taught that polyvinyl chloride will interact with a number of metal salts to produce colored products in thermal processes. Thermal imaging utilizing polyvinyl chloride and zinc oxide together with sensitizing additives, such as silver salts, diazonium salts, acids, etc., has been disclosed in U.S. Pat. Nos. 2,772,158; 3,107,174; 3,155,513; 2,789,052; 2,789,053 and 2,772,159.
We have now discovered that a mixture of a chlorinated polymer and a zinc salt, when utilized in effect as a paintable or printable composition, will perform exceptionally well as a thermochromic indicator for heat recoverable articles.
Evidence has been provided in the literature that a colored organometallic complex will be formed when mixtures of polyvinyl chloride and zinc oxide are heated to their degradation point. Furthermore, it is also known that zinc oxide can function as an effective stabilizer for polyvinyl chloride at temperatures below this, acting as a scavenger for hydrogen chloride produced during thermal decomposition. We have found that the combination of these two effects produces a severely abrupt change from white to black when a mixture thereof is heated to its degradation temperature, thus providing ideal thermochromic indication for use with heat reactive articles.
Furthermore, we have found that when such mixtures are used as temperature indicators, optionally additionally containing a binder polymer, the drawbacks noted above relative to prior thermochromic materials have been overcome. For example, there is no moisture sensitive or water soluble element present therein. Unlike the prior art inorganic thermochromics, no reversion to white occurs upon standing after the color change has been initially attained. Where zinc oxide is used, it is a pure white, having a higher refractive index than prior art organic thermochromics, and can be commercially obtained in an optimum particle size for excellent dispersion and hiding power. Upon conversion, the composition converts to a very deep black so that maximum contrast is visually apparent. The temperature of conversion is ideal for many types of heat recoverable articles, and little or no heat reactive substrate degradation is observed upon thermal conversion of the composition.